Page 344 - Practical Ship Design

Structural Design 30 1

(i) avoiding or minimising structural discontinuities,
(ii) avoiding or minimising vibration,
(iii) reducing corrosion and facilitating maintenance and repairs.

10.6.1 Avoiding, minimising or compensating for structural discontinuities

Some of the best possibilities of minimising structural discontinuities occur when
the outline general arrangement is being drawn as decisions taken at this time
determine the type, extent and positioning of erections, the ends of which will form
some of the most severe discontinuities.
Whilst a forecastle is likely to be necessary from a seakeeping point of view on
all except a very large ship, a poop is only likely to be required on a fairly small
vessel. However, the ends of both forecastles and poops are well away from
amidships and do not generally present any significant structural problem.
A side-to-side erection near midships such as a bridge should be avoided if
possible, but if it is a necessary arrangement feature then it should be made as long
as possible for two reasons:
(i) so that its ends are as far away from amidships as practicable thus
minimising the stress at the discontinuity, and
(ii) the maximum value is obtained from its contribution to longitudinal
strength.
The junction of the strength deck and the ship side plating occurs at the point of
maximum stress due to longitudinal bending. On large ships local stresses at this
connection are now greatly reduced by the practice of fitting a radiused gunwale
plate. The danger of cracking at this point is recognised and reduced by the
requirement of Classification Societies for construction in this region to be of
notch tough steel.
The use of notch tough steel of course goes well beyond this immediate area with
Classification Societies requiring the use of the different grades “D’ and “E’ in
various stressed positions depending partly on ship size and partly on plating
thickness.
It is also most important that welded attachments to stressed plating of this sort
of such items as stanchions and fairleads are kept to a minimum (preferably none).
A similar situation arises at the bilge which is again a highly stressed area, and
great care should be taken with the detail design of the attachment of the bilge keel
to the hull plating.
Local discontinuities come from many other features; sometimes these dis-
continuities must be accepted and compensated for, in other cases they can be
eliminated by wise design.